This invention relates to a light-mixing compartment for a photographic projector particularly for photographic copying or enlarging apparatus.
German Patent Application P 25 46 721.4, corresponding to U.S. Pat. No. 4,053,222 relates to illuminating equipment having a two-part light-mixing compartment in which the light admitting (first) portion is in the form of a truncated pyramid and the light-emerging (second) portion has the shape of a parallelepiped with side faces either disposed perpendicular to the light emerging area or with side faces inclined at a different angle from the inclination of the side faces of the first portion.
The inclination of the side faces of the light-mixing compartment controls the light distribution in the light-emerging area, so that it is possible to obtain an extremely uniform brightness distribution over the entire illuminating surface without the need for tapered diffusing screens, and it is also possible to utilize a diffusing screen over the light emerging area having a uniform thickness over its entire area.
The diffusing screen in the light emergence area of the light-mixing compartment equalizes the slight irregularities in the mixed and distributed light, which still remain even in the proposed illuminating equipment. To increase the light yield of illuminating equipment of the previously-mentioned type, the length of the light-mixing paths is maintained as short as possible and the thinnest possible diffusing screen or none at all is utilized.
If a thinner diffusing screen with a slighter diffusing effect is used, instead of the previously discussed diffusing screen at the light emergence area of the light-mixing compartment, this causes an intolerable contrast in brightness at the margins or edges of the illuminated format. It is, therefore, not possible to increase the light yield of the previously discussed illuminating equipment by reducing the diffusing effect in the light emergence area. An object of this invention is, therefore, to improve the light yield obtained from the aforementioned type of illuminating equipment by selecting a particular shape for the sides of the light mixing compartment.